1. Field of the Invention
The present invention relates to a variable valve lift apparatus. More particularly, the present invention relates to a variable valve lift apparatus that can control valve lift according to engine operation condition.
2. Description of Related Art
An internal combustion engine generates power by burning fuel in a combustion chamber in air media drawn into the chamber. Intake valves are operated by a camshaft in order to intake the air, and the air is drawn into the combustion chamber while the intake valves are open. In addition, exhaust valves are operated by the camshaft, and a combustion gas is exhausted from the combustion chamber while the exhaust valves are open.
An optimal operation of the intake valves and the exhaust valves depends on a rotation speed of the engine. That is, an optimal lift or optimal opening/closing timing of the valves depends on the rotation speed of the engine. In order to achieve such an optimal valve operation depending on the rotation speed of the engine, various researches, such as designing of a plurality of cam and a variable valve lift (VVL) that can change valve lift according to engine speed, have been undertaken.
A variable valve lift apparatus using hydraulic pressure has been widely researched, and the variable valve lift apparatus may control closing timing of a valve by controlling releasing timing of hydraulic pressure as well as control valve lift by using hydraulic pressure. However, when oil temperature of the variable valve lift apparatus is not within an oil operation temperature range, supplying or releasing of hydraulic pressure can be unstable.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.